optimizer_dynamic_sampling-优快云博客

本文探讨了Oracle数据库中成本基于优化器(CBO)在处理同表中相关联的两个列时可能出现的问题，并介绍了如何通过调整dynamic_sampling参数或使用dynamic_sampling hint来改进CBO的准确性。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

看到这样一段话：

Correlation between columns in the same table always causes problems if those columns appear together in your where clause; the issue is not restricted to indexes. Sometimes you will be able to work around the problem by using the optimizer_dynamic_sampling parameter, or the dynamic_sampling hint (both of which appeared at some stage in 9i) to instruct Oracle to take a run-time sample of 32 or more blocks fromcritical tables to see what fraction of the rows matches your where clause.

同表中相关联的2个列，也会导致CBO计算错误，所以要使用dynamic_sampling这个参数，下面转一篇介绍这个参数的文章

http://space.itpub.net/35489/viewspace-592375

dynamic_sampling也可作为一个hint：

select

/*+ index(t1) dynamic_sampling(t1 1) */

small_vc

from

where

ind_pad = rpad('x',40)

and n1 = 2

and n2 = 2

;